Incident Overview
On October 20, 2012, at around 19:42 Japan Standard Time, a Boeing 737-800 registered as JA342J, operated by JAL Express Co., Ltd., was climbing after takeoff from Tokyo International Airport for Matsuyama Airport on a scheduled flight. At an altitude of approximately 13,000 ft and about 37 km west of Tokyo International Airport, the flight crew observed a decrease in the No.1 engine's rotating speed and an increase in exhaust gas temperature. The engine was shut down, and the aircraft returned to Tokyo International Airport, landing at 20:10 after receiving priority from air traffic control. The aircraft carried 144 occupants, including a pilot-in-command, five crew members, and 138 passengers. No injuries were reported.
Damage to Aircraft
The aircraft sustained slight damage, with major damage to the interior of the No.1 engine. The engine, a CFM International CFM56-7B24E two-spool turbofan, exhibited extensive damage to its high-pressure compressor (HPC). The fourth stage HPC blades had dents on their trailing edges. All fifth stage HPC blades were fractured from their roots. The sixth and subsequent stage blades, the combustion chamber, and turbines showed damage from high temperatures, including burnout.
Investigation Findings
The investigation by the Japan Transport Safety Board (JTSB) revealed a scabbed contact trace (SCAB) at the bottom inside the fifth stage of the HPC casing, about four times thicker than normal. Material analysis confirmed the SCAB originated from the fifth stage blades. Traces of dried liquid, mainly water, were found in cavities near the bottom of the HPC casing and in lower bleed air ducts. The manufacturer determined that water could accumulate in these cavities if it infiltrated near the fifth stage. A simplified analysis indicated that accumulated water might suppress local thermal expansion of the HPC casing during engine start, reducing clearance between blade tips and casing. The aircraft had been parked overnight at the airport without engine inlet covers, and precipitation occurred prior to the incident.
Analysis
Fracture surface examination of the fifth stage blades showed traces of heavy load and fatigue crack propagation. The manufacturer's load test using a simplified test piece suggested that initial cracks could lead to fracture after approximately two representative flight cycles. The investigation concluded that the blade fractures likely resulted from all fifth stage blade tips contacting the casing, applying heavy loads to the blade roots and initiating cracks. The cracks then propagated under cyclic flight loads, eventually causing blade airfoil separations leading to cascading failures.